Most fuel systems for internal combustion engines include a tank for holding a supply of fuel, an engine where the fuel is converted into energy, and a filter between the tank and the engine to separate particles and contaminants that could cause adverse effects to the engine. In certain engines, for example diesel engines, the fuel can wax or plug the filter during cold operation. This is a well-known problem, and many devices have been developed in an attempt to solve this. For example, a heater can be provided internally of the filter housing to heat the fuel and the media as the fuel passes through the filter (see, e.g., Richard, U.S. Pat. No. 4,091,265).
Another technique which can be used alternatively or in addition to a filter heater, is to use excess fuel from the engine. Excess fuel that is not burned in the engine is normally recirculated back to the tank. Since the fuel is warmed when it passes through the engine, the fuel can be directed back through the filter to warm the filter. It is believed that prior techniques have included a simple valve that opens under cold temperature operating conditions, and redirects at least a portion of the excess heated fuel back into the filter element. The valve then closes after the engine is warm to direct all the excess fuel back to the tank. While this technique provides some relief from the problem of waxing and plugging of the filter, it is believed that the prior techniques have also recirculated any excess air received back from the engine. The engine supplies air particularly during its priming, and the air is simply fed back with the fuel through the filter to the engine. Recirculating the air with the fuel, however, can effect the combustion characteristics of the engine, which is undesirable from an efficiency standpoint.
As such, applicants believe there is a demand in the industry for a fuel system which recirculates excess warm fuel from the engine so as to at least reduce waxing and plugging problems in the filter during cold start, but which can also differentiate between fuel and air, such that only the warm fuel is directed back to the filter during cold star-up, and any air is directed back to the tank.